The invention relates to a power circuit for starting and operating a gas discharge lamp with ac-voltage and, in particular, to a power circuit for starting and operating a gas discharge lamp with ac-voltage where the power circuit is provided with:    at least a first and a second lamp output terminal between which, in use, the gas discharge lamp is connected,    a dc-voltage input, with at least two input terminals, a series connection of at least two switching elements such as, for instance, power transistors, which is connected between the input terminals of the dc-voltage input,    a control circuit arranged for delivering control pulses to the switching elements for bringing those switching elements alternately and without overlap into conduction,    at least one lamp coil which is connected on the one side to the junction of the switching elements and is connected on the other side to the first lamp output terminal,    at least one resonance capacity, at least comprising one or more capacitors, the resonance capacity being connected, on the one side, to the first lamp output terminal and being connected, on the other side, to one or both input terminals,    at least one coupling capacity, at least comprising one or more capacitors, the coupling capacity having a capacity value which is considerably greater than the capacity value of the resonance capacity, the coupling capacity being connected, on the one side, to the second lamp output terminal and being connected, on the other side, to one or both terminals of the dc-voltage input, and    a variable frequency oscillator which, in use, delivers a signal to the control circuit with a frequency which is determinative of the control frequency with which the switching elements are brought into conduction, periodically and without overlap.
Known electronic power circuits for gas discharge lamps are typically built up as indicated in FIG. 1. The ac-voltage, originating from an ac-voltage source 1, typically the voltage furnished by the public electricity grid, is rectified by rectifier 4 after high-frequency filtering by filter 3, and then converted by a Power Factor Corrector 5 to a smoothed dc-voltage. Here, Power Factor Corrector 5 ensures that the current supplied by the voltage source 1 to the power circuit meets the requirements applying in respect of harmonic currents for lighting appliances.
The dc-voltage obtained in the manner described is converted by two switching elements such as, for instance, power transistors 6a and 6b, which are driven by control circuit 7, to an ac-voltage with a much higher frequency than the supply ac-voltage from source 1. The thus formed block-shaped or, when voltage rate limiting capacities 6c and 6d are present and when power transistors 6a and 6b conduct in a less than overlapping manner, trapezium-shaped ac-voltage is passed, via an LC-section consisting of self induction 8, further to be called lamp coil 8 and capacity 9, further to be called resonance capacity, to one electrode of the lamp 10 to be supplied. The second electrode of lamp is connected, via a coupling capacitor 11, to one of the dc-voltage outputs of Power Factor Corrector 5.
For starting the lamps, the control frequency for power transistors is selected such that it is in the vicinity of the resonance frequency of the output circuit, formed by lamp coil 8 and resonance capacity 9, so that across this output circuit a sufficiently high voltage is built up to have the connected lamp ignite.
With the customary circuits for power circuits, as described in the introduction, the LC-circuit, consisting of lamp coil 8 and resonance capacity 9, is designed to be relatively low-ohmic with a characteristic impedance Z0=sqrt (L(8)/C(9)), which is in the same order of magnitude as the high-frequency compensating resistance with which a gas discharge lamp in stable, high-frequency operation can be modeled. This leads to a relatively large coil and to large currents through the coil and the power transistors at ignition of the lamps.
Consequently, the load on the transistors in the ignition stage of the lamp is rather high and reliability problems may occur, or very costly power transistors are required, while, also in normal operation, the losses in the lamp coil are rather large, so that the internal temperature of the power circuit becomes high, or relatively expensive constructions are to be used to keep the temperature sufficiently low. This is of interest particularly with regard to the presence of electrolytic capacitors in the power circuit, which, at a high temperature, have a very short life span.
A variant on the power circuits described hereinabove is known from U.S. Pat. No. 5,914,571, wherein for the lamp ignition of the high intensity gas discharge lamp, use is made of a resonant circuit operating on the third harmonic of the control frequency, and, for normal operation, a resonance capacity is included in series with the lamp coil. With this solution the height of the ignition voltage is determined by the damping in the resonance circuit effecting the ignition. The height of the ignition voltage is mostly limited by magnetic saturation phenomena in the lamp coil. Further, the charge on the switching transistor remains quite high, in that, in such circuits, at least in a large part of the lamp ignition phase large ‘shoot through’ currents occur in the series connection of the power transistors, as a result of the recovery process occurring in the diodes at the moment the power transistors switch on. This is the case because at the moment one of the transistors switches on, the current still runs through the anti-parallel diode of the other transistor. This leads to additional losses in the switching transistors, and, in certain types of transistors also to a reduced reliability. Further, in the circuit according to U.S. Pat. No. 5,914,571, in normal operation when the gas discharge lamp is supplied via the series resonant circuit formed by the lamp coil and the additional resonant capacity (not shown in FIG. 1), the voltage across the lamp coil is relatively high, leading to a relative large coil with relatively much loss, having the above-mentioned drawbacks.
In the existing electronic power circuits, the losses are quite large particularly in the coil connected in series with the lamp, as set forth hereinabove.
Without additional costly measures, this has an adverse effect on the electrolytic capacitor included in the Power Factor Corrector, which capacitor, due to the high temperatures for some uses, has too short a life span so that it is difficult to construct a compact electronic power circuit. The fact is that the internal temperature of the electrolytic capacitor, determined by the internal temperature in the electronic power circuit, enhanced with the temperature increase resulting from the ac-voltage charge of the electrolytic capacitor, is determinative to the life span of this type of capacitor. To this it can be added that the electrolytic capacitor in the known electronic power circuits undergoes a relatively high ac-voltage charge in that ac-voltage from the converter of the Power Factor Corrector as well as from the dc-voltage to ac-voltage converter feeding the lamps, run through this capacitor. This causes additional internal temperature increase of the electrolytic capacitor and a further shortening of the life span of this capacitor.
The result is that the existing electronic power circuits become defective, sometimes after only a few years of operation, in particular in uses when they operate continuously, therefore for 168 hours a week, or almost continuously.
Further, especially with older gas discharge lamps, the risk of the rectifying effect in the gas discharge lamp is present. Under certain circumstances, this rectifying effect may cause the electronic power circuit to become defective.
Further, in most known electronic power circuits, the lamp output depends on the condition of the gas discharge lamp. With high intensity gas discharge lamps, this can change as a result of change in the emission properties of the electrodes, especially caused by the electrodes burning down, so that they become shorter during the life span of the lamp. With low intensity gas discharge lamps, without additional measures, the ambient temperature of the lamp plays a large part in the power consumption of the lamp. When used for illuminating purposes, a constant light output is desirable, while in uses wherein ultraviolet radiation of the gas discharge lamp is used for water purification, by utilizing the bactericidal action of the UV-radiation, a constant amount of emitted UV-radiation is desirable. This latter can be achieved by stabilizing the lamp output. Moreover, it can be desirable to be able to reduce the lamp output to save power or to lengthen the life span of the lamp or the life span of the electronic power circuit. In some uses, stabilizing the lamp current through the gas discharge lamp can be desirable instead of stabilizing the lamp output, for instance in connection with the life span of a special construction of the lamp electrodes.
Further, existing electronic power circuits are often not suitable for igniting gas discharge lamps via long connecting wires, because the wiring capacity of the connecting wires affects the resonant circuit used for the ignition of the lamp such that the ignition voltage required for a reliable ignition is no longer achieved.
Another drawback of the known electronic power circuits is that feeding high intensity gas discharge lamps takes place at a frequency at which acoustic resonances can occur in the lamp, which may shorten the life span of the lamp and lead to troublesome “light twinkling phenomena” in the lamp.